High pressure systems are routinely employed in analytical laboratories, for example, in the isolation and characterization of a particular compound. As the demand for high pressure systems continues, the demand for high pressure seals follows.
At present, compression ferrules are used fluid line connectors in high pressure systems such as high performance liquid chromatography. These compression ferrules are relatively small structures that cooperate with one or two-piece tube fittings. These devices require precise machining of detailed ports where two fluid lines connect together, or where the fluid lines interconnect to components such as fluid manifolds and valves. As the number of these fluid interconnections increase the consequential cost of machined parts and the cost of production tooling increases.
Depending on frequency of use, the high pressure systems can exert a tremendous amount of stress on the fluid connects. The seal that is formed at the connection site can begin to deteriorate due to this stress.
The present fittings comprise a conical ferrule and a compression nut which are proximate to the tube end. The mating part has a threaded and conical machined recess at the receiving port which cooperates with the tube end compression nut and ferrule to make a seal. As the compression nut is tightened into threads of the machined port it forces the ferrule forward. This forward motion presses the conical ferrule end against the conical recess which causes the front edge of the ferrule to reduce in size until it fits tightly around the tube. The seal is achieved by the compression deformation of the ferrule by the port cone and by the compression deformation of the ferrule against the tube outside surface. Modern chromatographic system pressures are becoming greater than the present fittings are designed to sustain. The greater deformations needed to achieve sealing results in reduced reliability of the seals.
When the present fittings are assembled they sometimes have a characteristic which is detrimental to the quality of the chromatographic separation. There can be a cavity left between the tip of the tube and the bottom of the port. This cavity will retain fluids and can cause an unwanted mixing of fluid species during chromatographic separation. Minimizing this cavity depends on the skill of the person who makes-up the fitting as the ferrule closes on the tube. If the tube is held tightly into the port as the compression nut is tightened against the ferrule, the tube will be pressed forward while the ferrule collapses against it in order to close the tip gap.
The present fittings, once set, often cannot be interchanged among various ports because of the dead volume issue. When the ferrule is compressed on the tube at the first assembly the distance from the ferrule to the tube tip is unique for that port. If the tube and fitting is removed and used in a different port there may be a space left at the tube tip. This can adversely impact the chromatography when the tube carries fluids which should not be mixed.
There are high pressure seal assemblies that attempt to mitigate the problems described above. For example, there is disclosed a high pressure tube seating assembly wherein the seal is formed at a contact interface between a spherical distal end portion of a tube and a receiving surface. It is believed that as greater axial force is applied, the spherical distal portion deforms to more closely approximate the geometry of the receiving surface. However, in actuality, as greater axial pressure is applied, the spherical distal portion deforms but does not deform in such as manner as to significantly compliment the receiving surface.
In modern chromatographic systems pressures are being increased and the internal fluid volumes are being reduced. Therefore the reliability and seal characteristics of present fittings are becoming problematic. The present fitting has two seal surfaces which are both relatively larger than the fluid path diameter they are sealing. Those seal surfaces are remote from the tube tip and port opening so there is opportunity for fluid “dead-volume”. As the pressure is raised the compression nut force and resulting distortion of the ferrule and related parts becomes excessive. As the system internal fluid volume becomes smaller the fitting dead-volume and especially sensitivity to the assemblers skill become major impediments to chromatographic quality.
There exists a current to have a high pressure fitting assembly that upon axial pressure will deform in such a manner as to compliment the receiving element, thereby, forming a high pressure seal with optimum fluid integrity.